Synergistic effect of Cu and Cr on pitting behavior induced by MnS inclusions in low alloy steels

We designed three low alloy steels containing different Cu and Cr contents to investigate the synergistic effect of Cu and Cr on pitting initiation in low alloy steels. A metallographic analysis of inclusions in steels and electrolytic extracted inclusions was conducted using scanning electron microscopy (SEM)-energy dispersive X-ray spectroscopy (EDS) techniques. First-principles calculations were also performed to reveal the segregation behaviors of Cu and Cr at the α-Fe/MnS interface. Preferential dissolution of MnS was observed by in-situ ex-environment SEM. It was seen that severe pitting occurred in the steel containing both Cu and Cr. Statistical results indicated that Cu and Cr stimulate each other to accumulate on sulfides, while the stimulating effect of Cu on Cr is stronger than that of Cr on Cu. This synergistic effect intensifies the pitting corrosion of the steel. Experimental results were consistent with those of the first-principles calculations. From these results, a proposed pitting initiation mechanism by the synergistic effect of Cu and Cr on MnS inclusions is described.